(1) Field of the Invention
The invention relates in general to handovers in cellular networks. In particular it relates to transmitting data on one frequency and performing measurements on another frequency during or for an interfrequency handover.
(2) Description of Related Art including Information Disclosed Under 37 CFR 1.97 and 1.98.
In cellular networks, where the communication connections are separated from each other using code division multiple access (CDMA) technique, a mobile station having an active communication connection with the cellular network should be able to receive data at the radio frequency related to that communication connection practically all the time. In an interfrequency handover the frequency on which an active communication connection exists is changed. A cell change may accompany the interfrequency handover, in which case the maneuver is an intercell-interfrequency handover, or the frequency change may take place within a single cell meaning that an intracell-interfrequency handover is performed. The present invention is equally applicable to all interfrequency handover types. During an interfrequency handover, a mobile station should be able to receive data on the first frequency and simultaneously to perform measurements and/or receive data on a second frequency.
A mobile station, which has two receivers, may simultaneously listen to two frequencies. For allowing a mobile station, which has only one receiver, to receive data related to the active communication connection uninterruptedly on a first frequency and to receive data also on a second frequency, transmission gaps can be left to the radio transmission on the first frequency. During the transmission gaps, no data is transmitted to the mobile station using the first frequency. Compressed mode transmission refers to a transmitting data in such a way there are breaks (transmission gaps) in the transmission.
Usually data to be transmitted over a radio interface is processed in such a way that the actually transmitted data has more redundancy than the original data. This way it is possible, for example, to detect transmission errors and to recover from them. Especially when the data to be transmitted is related to a real-time application, it may be desirable to transmit the user data at an unchanged data rate even during a compressed mode transmission. In this case a compromise usually has to be made for ensuring, on the one hand, the quality of the transmitted data and, on the other hand, enough time for listening to radio transmission on a second frequency.
Typically data is transmitted over the radio interface in frames, which have a certain number of time slots. The time slots comprise a certain number of symbols. The number of time slots in a frame, the number of symbols in a time slot and the duration of a symbol are usually defined in the applicable cellular system specifications. For example, the Universal Terrestrial Radio Access network (UTRA) of the Universal Mobile Telecommunication System (UMTS) employs 15 time slots in each frame in the UTRA Frequency Division Duplex (FDD) system. UTRA FDD employs CDMA technique.
FIG. 1 illustrates a sequence 100 of frames during a continuous mode transmission. The frames follow immediately each other in time. Sequence 101 in FIG. 1 presents an example of a compressed mode transmission. In sequence 101, the transmission of frames number N and N+2 lasts as long as the transmission of frames in the continuous transmission. The transmission of frames number N+1 and N+3 in sequence 101 lasts a shorter time than that of frames N and N+2 in the same sequence. The frames N+1 and N+3,whose transmission takes a shorter time, may carry a smaller amount of user data as frames N and N+2. It is also possible that all frames in compressed mode carry the same amount of user data.
Usually the compressed mode transmission lasts many frames. FIG. 2 illustrates an example of periodically repeated transmission gaps 211 according to UTRA specification 3G TS 25.215, Physical layer measurements. The transmission gap length (TGL) is the duration of the transmission gaps 211. Usually TGL is expressed in numbers of time slots. According to 3G TS 25.215 specification, there are up to two transmission gaps within a transmission gap period (TGP). The repeated transmission gap periods are presented in FIG. 2 with rectangulars 220a, 220b and 220c. The transmission gaps within a transmission period are separated from each other by a transmission gap distance (TGD) . The duration of the transmission gap period is an integer number of frames, and the duration of the transmission gap distance is an integer number of time slots. During the compressed mode operation, the transmission gap period is repeated for a certain number of times, and the pattern duration (PD) is a multiple of the number of frames in one TGP.
A system frame number (SFN) is the parameter specifying the frame in which compressed mode transmission starts. The slot number (SN) specifies the time slot in which the first transmission gap within a transmission gap period starts. A cellular network can tell to a mobile station the frames where transmission gaps are by, for example, signaling the values for SFN, SN, PD, TGP, TGD and TGL to the mobile station. It is also possible to define the transmission gap pattern using other parameters, but this set of parameters, which complies with the 3G TS 25.215 specification, is used here as an example.
According to 3G TS 25.215 specification, within a transmission pattern two transmission gap periods having different durations can be repeated alternatingly. Parameter TGP1 defines the duration of the odd-numbered transmission gap periods, and parameter TGP2 defines the duration of the even-numbered transmission gap periods. All transmission gap periods are similar from the beginning of the transmission gap period to the end of the second transmission gap within a transmission gap period (or to the end of the only transmission gap, if there is only one transmission gap within each transmission gap period). The difference in the transmission gap periods having a first duration TGP1 and those having a second duration TGP2 is that in the end of the longer transmission periods there are more frames, which are similar to those transmitted during continuous operation. If only one value duration TGP of the transmission gap period is defined, then all transmission gap periods have this duration.
In a handover situation it is important that the mobile station can receive synchronization information from the target cell. In UTRA FDD, for example, the synchronization channel (SCH) is the logical channel that carries this information, and physically there are certain synchronization symbols in each time slot. The synchronization symbols of a frame indicate, in addition to the timing of the transmission, the long scrambling code group which the target cell is using for downlink transmissions. The long scrambling codes are grouped into a certain number of groups, and each group has a certain number of scrambling codes. For successfully receiving control information from the target cell, the mobile station has to find out the long scrambling code of that cell. The larger number of synchronization symbols which the mobile station can receive from the target cell, the larger the probability to successfully determine the long scrambling code.
The periodical compressed mode enables the determination of certain number of synchronization symbols. The length and position of the transmission gap defines the indexes of the time slots (in the target cell) whose synchronization symbols the mobile station can receive. It is advisable to choose the transmission gap distance so that the as many time slot indexes as possible are selected. The repetition of the transmission gap pattern allows the synchronization symbols to be received multiple times, and thus the value of the symbols can be determined more accurately than based just on receipt of the symbols.
When user data is transmitted over the radio interface, it is typically first coded (to increase redundancy and resistance to bit errors in transmission) and then interleaved (to increase resistance to bursty transmission errors). The coding and interleaving are usually done in the first protocol layer. There are at least three ways to create the transmission gaps. The first alternative is to limit the amount of user data delivered from the upper protocol layers to the first protocol layer. This approach does not work for delay-sensitive applications, such as real-time applications, where there is no time, for example, to buffer the data. A second alternative to create a transmission gap is to reduce the spreading factor used to spread the data of the communication connection according to the CDMA technique. Symbols carry an information stream whose rate is the chip rate divided by the spreading factor. Reducing the spreading factor by two means that the symbol rate of the information stream is doubled. This means that it is possible to carry the same amount of user data in half of the time slots. A third alternative to create a transmission gap is to puncture the coded data so that the rate of the coded data is less in the compressed mode than in the continuous transmission mode. Rate matching is usually performed between coding and interleaving. Rate matching means either repeating certain selected bits of the coded data or ignoring certain selected bits of the data, in order to produce a coded data flow having a certain rate. Puncturing refers to ignoring certain bits of the coded data. Using puncturing, it is possible to carry the same amount of user data in all frames, despite of the transmission gaps. There is a certain maximum duration of a transmission gap that is feasible to create using puncturing. If too much bits of the coded data are punctured, the quality of the transmission deteriorates drastically.
For data related to real-time applications, it is thus possible create transmission gaps by reducing the spreading factor or by puncturing the coded data. In general, the transmission power of the frames, during which the transmission gap occurs, needs to be increased to ensure the quality of the transmission, when puncturing or reduction of the spreading factor is used to create the transmission gaps.
Reducing the spreading factor by two means that the transmission gap length can be 7 time slots in a system where there are 15 time slots per frame. 3G TS 25.215 specification allows one or two transmission gaps of 7 time slots to be placed in isolation (i.e. one or two transmission gaps of 7 time slots within a transmission gap period), or two transmission gaps may be placed next to each other in two consequent frames within a transmission gap period. Using the latter double frame approach, it is thus possible to have within a transmission gap period one transmission gap of 14 time slots. The switching of the receiver from a frequency to another frequency and back may take a time of about one or two time slots. Table 1 presents the number of synchronization symbols, which are transmitted by the target cell and which the mobile station can capture, when transmission gaps are created by reducing the spreading factor by two.
TABLE 1Number of captured synchronization symbols when transmission gaps arecreated by reducing the spreading factor by two.Number of captured syn-Transmission gap durationSwitching timechronization symbols 7 time slots1 time slot2*(7 − 1) =122 time slots2*(7 − 2) =1014 time slots1 time slot14 − 1 =132 time slots14 − 2 =12In UTRA FDD, each cell has a primary scrambling code which is used as long as there are available channelization codes related to said primary scrambling code. The channelization codes are orthogonal and their spreading factor varies typically from 4 to 512 chips per user data bit. Each downlink communication connection is given a specific channelization code. The use of a channelization code having a small spreading factor prevents the use of a certain number of channelization codes having a larger spreading factor. When creating transmission gaps by reducing the spreading factor by two, there may occur a situation, where it is not possible to change a first channelization code to a second channelization code whose spreading factor is smaller, because there are not enough free channelization codes whose spreading factor is smaller. This situation is usually called code limited.
In a code limited situation it is possible to reduce the spreading factor by two by using a secondary scrambling code with the new channelization code; see TSGRI#7(99)b27, Ericsson: “Use of multiple scrambling codes in compressed mode” TSG-RAN Working Group 1 meeting 7, Hannover, Germany, Aug. 30–Sep. 3, 1999. The problem is using a secondary scrambling code is that the orthogonality of the channelization codes with a cell is lost. The interference caused by the transmission in the tow cell Pintra is increased compared to the interference caused by the surrounding cell Pinter. The target value for the signal-to-interference (SIR) in the transmission power control has to be increased considerably to ensure the quality of the transmission. As can be seen in Table 2, the required increase in the target value for SIR depends on the ratio Pintra/Pinter and on the channel impulse response profile, which defines the orthogonality factor for the primary scrambling code. When the own cell interference is about the same as the interference caused by surrounding cells, i.e. Pintra/Pinter=0 dB, the increase in the target SIR value is smaller that when Pintra/Pinter is larger, i.e. when the mobile station is nearer the base station. A 3 dB increase in the target value for SIR is due to reduction of the spreading factor by two.
TABLE 2Required increase in the target value of SIR when a secondary scramblingcode is taken into usePintra/PinterIncrease in target SIRIndoor10 dB4.7 dB + 3 dB = 7.7 dB 5 dB2.5 dB + 3 dB = 5.5 dB 0 dB0.9 dB + 3 dB = 3.9 dBVehicular10 dB3.7 dB + 3 dB = 6.7 dB 5 dB2.7 dB + 3 dB = 5.7 dB 0 dB1.6 dB + 3 dB = 4.6 dB
Creating transmission gaps by reducing the spreading factor by two may thus cause many problems in a code limited situation. Firstly, the transmission power of certain frames during the compressed mode transmission has to be increased, and it has to be increased typically more than 4 dB. This causes more interference to the other transmissions in the cell. In addition, in a code limited situation the base station cannot necessarily increase the transmission power of the compressed mode transmission as much as required because of all the other active communication connections. Secondly, a required increase for the target value of SIR needs to be estimated. This is difficult, because the increase in SIR depends on the position and velocity of the mobile station and because it is not possible to measure the ratio Pintra/Pinter. If the increase in SIR is always chosen to be large enough, for example 7.7 dB, to ensure a successful interfrequency handover, then unnecessary interference is caused at least in some cases.
It is possible to use puncturing for creating transmission gaps. The transmission power of the frames which contain the transmission gaps needs to be increases also in this case. The 3G TS 25.215 specification allows transmission gaps whose length is 7 time slots for interfrequency handover. It is not feasible to create this long transmission gaps using puncturing, because the quality of transmitted data deteriorates. Table 3 presents the estimated increase in the target SIR when puncturing is used to create transmission gaps, whose length is 5 time slots. Compressed transmission in 10 time slots instead of 15 time slots causes a 1.7 dB increase to the target values of SIR.
TABLE 3Required increase in the target value of SIR when puncturing is usedPintra/PinterCodingIncrease in target SIRPedestrian6 dBConvolutional1.0 dB + 1.7 dB = 2.7 dB6 dBTurbo0.5 dB + 1.7 dB = 2.2 dBVehicular6 dBConvolutional2.0 dB + 1.7 dB = 3.7 dB6 dBTurbo1.5 dB + 1.7 dB = 3.2 dB
When puncturing is used, the compressed transmission may use the primary scrambling code. The interference cause by the own cell is roughly the same thoughout the cell, and therefore only one value for the ratio Pintra/Pinter is shown in Table 3. The increase in the target value of SIR is less than when the spreading factor is reduced. The increase in the target value of SIR depends on the channel model and on the velocity of the mobile station, but even if the largest value for the increase in Table 3 is 3.7 dB. If turbo coding, which is less sensitive to puncturing and/or transmission errors than convolutional coding, is used in the compressed transmission, even a smaller increase in target SIR is enough.
In a code limited situation using puncturing to create transmission gaps causes a smaller increase in transmission power than reducing the spreading factor. The problem in puncturing is that it is not possible to capture enough synchronization symbols on the second frequency. Table 4 shows the number of captured synchronization symbols. At maximum 9 synchronization symbols can be captured using the double frame method. This provides a much smaller probability for determining the scrambling code group, and further a smaller probability for carrying out a successful handover, than the 12 synchronization symbols that can be determined when transmission gaps are created by reducing the spreading factor by two (see Table 1). Thus, although puncturing is preferred over reduction of spreading factor from the view-point of transmission power, its use in not feasible.
TABLE 4Number of captured synchronization symbols when transmission gaps arecreated by puncturing.Number of captured syn-Transmission gap durationSwitching timechronization symbols 5 time slots1 time slot2*(5 − 1) = 82 time slots2*(5 − 2) = 610 time slots1 time slot10 − 1 = 92 time slots10 − 2 = 8
The object of the invention is to present a flexible method for preparing an interfrequency handover. A further object of the invention is to present a method using which an adequate number of synchronization symbols can be captured when the transmission gaps are created by puncturing. Even a further object of the invention is to present a method, which can be supported in the existing systems with small modifications.
The objects of the invention are achieved by letting the transmission gaps have different durations during an interfrequency handover.